FIG. 1 shows an audio frame format (i.e., a frame) associated with standards promulgated by the Motion Picture Experts Group (MPEG) for audio compression and decompression. It will be understood that the term MPEG is also used herein to refer to various for different types of compression and decompression. For example, MPEG1 and MPEG2 are formats promulgated by the Motion Picture Experts Group.
Referring to FIG. 1, the first field is a sync word, which is used to identify the start (or end) of the audio frame. The sync word has a value equal to twelve successive binary digits (or bits) equal to a “1.” The sync word value can also be represented by a hexadecimal value of 0xFFF (1111 1111 1111) where, as understood by those skilled in the art, “0x” denotes that the following numbers are in hexadecimal format.
The sync word is followed by a frame header, which includes an identification field ID (1), a layer field LAYER (2), a protection bit field PROTECTION BIT (1), a bitrate index field BITRATE INDEX (4), a sampling frequency field Fs (2), a padding bit field PADDING BIT (1), a private bit field PRIVATE BIT (1), a mode field MODE (2), a mode extension field MODE EXTENSION (2), a copy right field COPY RIGHT (1), an original/copy field ORIGINAL/COPY, and an emphasis field EMPHASIS (2). The number in the parenthesis indicates the number of bits included in each field. MPEG audio frame headers are discussed, for example, on the World Wide Web at codeproject.com/audio/MPEGAudioInfo.asp. These fields are well known in the art so that detailed explanations for the fields are omitted and only the fields related with the present invention are discussed below.
The sampling frequency field Fs is composed of two bits and represents a sampling frequency. In the case of MPEG Layer 3, possible sampling frequencies include 44.1 KHz, 48 KHz, 32 KHz and a reserved frequency when values of the sampling frequency field are 00, 01, 10 and 11, respectively.
The padding bit field PADDING BIT includes one bit and represents whether or not dummy data is included in the frame. When the value of the data in the padding bit field (i.e., the padding bit) is “0,” the frame is not padded with dummy data whereas if the padding bit is “1,” the frame is padded with dummy data.
In operation, a conventional decoder receives compressed audio data (or a “bitstream”) in a series of audio frames and decodes each to reproduce a corresponding audio signal. Accordingly, the conventional decoder searches for the sync word in order to read information from the compressed audio bitstream. However, the sync word value of 0xFFF may not be unique in the compressed audio bitstream in that the compressed audio bitstream can include data having a value equal to the same value as the sync word (i.e., 0xFFF). Accordingly, the conventional decoder may erroneously detect data as a valid sync word, which may increase the sync word detection failure rate.
To address this type of problem, a conventional decoder may refer to the padding bit to assist in the search for the valid sync word. However, if the padding bit is incorrect (e.g., damaged or incorrectly calculated) the conventional decoder may have difficultly in detecting the valid sync word.